Total hip replacement (THR) is currently the gold standard treatment in the management of severe hip osteoarthritis as well as other end-stage hip conditions. In THR, the femoral head and acetabulum are respectively replaced with an artificial femoral stem and acetabular cup. At the proximal end of the femoral stem is a neck that terminates in a spherical ball and is generally referred to as the femoral head. The spherical geometry of the artificial femoral head enables it to freely articulate with the patient's acetabulum or a prosthetic acetabular cup fixed into the patient's acetabulum. To facilitate fixation of the femoral stem into the femur, it typically includes a shank portion (in other words a stem) that is inserted into the medullary canal of the femur. Fixation can be via the use of bone cement or press-fitted with treated surface that promotes bone ingrowth or ongrowth. THR prosthesis are currently available as either a two to three component system. In the three component system, it includes a femoral stem shank, a femoral head and an acetabular cup.
The purpose of THR is to replace the diseased hip joint with THR implants that can simulate the hip joint so as to enable load bearing and ambulation. In the course of load bearing and different ambulation activities, the replacement hip is subjected to repeated large impact forces, ranging from 2 to 8 times the body weight. Over time, such impactful forces may lead to loosening of the hip implant, including the femoral stem component. The impact forces may result in micromotion at the cement implant interface for cemented femoral stem. For press-fit bone ingrowth stem, any deficiency in bone ingrowth together with such constant impact forces may also lead to micromotion at the bone implant interface. This micromotion may result in femoral stem mechanical loosening. As a result of the above, revision surgery may be necessary. Current hip prosthesis has little intrinsic ability to reduce this impact force during axial loading. Further, wear particles generated from the abrasive forces acting on the prosthetic head in relation to the prosthetic acetabular may also contribute to loosening of the implant by osteolysis. When osteolysis takes place, the supporting bone structure fixed to the implant may be resorbed and hence biological loosening may occur.
U.S. Pat. Nos. 6,248,132, 5,389,107 and 6,336,941 attempt to address the undesirable side effects of large impact force experienced at the artificial hip joint.
In U.S. Pat. No. 6,248,132, to avoid micromotion from the large impact force, the acetabular cup was allowed to be displaced in its local axial direction during the load bearing via wave spring washers. Its femoral stem was inserted into a L-shaped shield fixed into the femur.
In U.S. Pat. Nos. 5,389,107 and 6,336,941, a force damping mechanism via springs was provided at the artificial femoral head. The shock absorption ability of the artificial hip joints in U.S. Pat. Nos. 6,248,132, 5,389,107 and 6,336,941 may not be effective. This is because these artificial hip joints are only able to displace in the local axial direction of the femoral head or the prosthetic acetabular cup. However, in the implanted artificial hip joint, the resultant joint force exerted on the prosthetic femoral head is in the superior-inferior anatomical direction and not in the local axial direction of the prosthetic femoral head or acetabular cup. Thus, the motion of the femoral stem component moves mainly in the superior-inferior anatomical direction, and not diagonally into or out of the hip joint.
In addition, it is well established in the literature that when loosening occurs in the femoral stem, it may be along the shank region and may always be in the direction of the femoral axial axis which is almost parallel to the superior-inferior anatomical direction. U.S. Pat. Nos. 5,389,107 and 6,336,941 clearly do not discuss loosening of the femoral stem along the femoral axial axis or avoiding micromotion along the femoral axial direction.
For U.S. Pat. No. 6,248,132, its femoral stem was inserted into a L-shaped shield fixed into the femur. The arrangement merely prevents the femoral stem from moving up along the L-shaped protective shield by way of a saw tooth step features. The flaw in U.S. Pat. No. 6,246,132 is that firstly, it merely stops the femoral stem from migrating upwards. In other words, it allows the femoral stem to move downwards relative to the shield, and does not allow the prosthesis to return to its original position subsequently. Such arrangement does not appear to be useful to avoid micromotion or to prevent loosening of the femoral stem along the femoral axial axis. In addition, the saw tooth step features has poor inherent longevity.
Therefore, there exists a need to address at least some of the issues identified in the existing hip prosthesis device.